Our invention relates to solid dielectric capacitors and more particularly to a monolithic ceramic capacitor comprising a single or multiple layered ceramic body and at least two electrodes in contact therewith. The invention also pertains to a method of fabricating such capacitors.
Multilayered ceramic capacitors have long been known and used extensively which employ noble metals such as platinum and palladium as the electrode materials. Generally, for the fabrication of such capacitors, "green" (unsintered) dielectric sheets have first been prepared from the proportioned ingredients of a desired dielectric ceramic material in finely divided form. An electroconductive paste containing powdered platinum or palladium has then been "printed" on the green sheets in a desired pattern. Then a plurality of such printed green sheets have been stacked up, pressed together, and sintered in a temperature range of 1300.degree. to 1600.degree. C. in an oxidative atmosphere.
This conventional method makes possible the simultaneous firing (cosintering) of the dielectric ceramic layers and the film electrodes interleaved therewith. It is also an acknowledged advantage of this known method that the noble metal electrodes are totally unaffected by the high temperature sintering in an oxidative atmosphere. Offsetting all such advantages is the expensiveness of the noble metals, which add substantially to the manufacturing costs of the multilayered ceramic capacitors.
Wada et al., U.S. Pat. No. 4,610,969, assigned to the assignee of the instant application, suggests a solution to the above problem. It teaches dielectric ceramic compositions consisting of a major ingredient expressed by the formula, (Ba.sub.k-x M.sub.x)O.sub.k TiO.sub.2, where M is at least either of magnesium (Mg) and zinc (Zn), and additives consisting of lithium oxide (Li.sub.2 O) and silicon dioxide (SiO.sub.2). The compositions may, or may not, additionally include at least one metal oxide selected from among barium oxide (BaO), calcium oxide (CaO) and strontium oxide (SrO).
Another solution is found in Wada et al., U.S. Pat. No. 4,610,970, which proposes ceramic compositions whose major ingredient is expressed by the formula, (Ba.sub.k-x-y M.sub.x L.sub.y)O.sub.k TiO.sub.2, where M is at least either of Mg and Zn, and L is at least either of Sr and Ca. To this major ingredient are added Li.sub.2 O, SiO.sub.2 and, optionally, at least one other metal oxide selected from among BaO, CaO and SrO.
Wada et al., U.S. Pat. No. 4,610,971 suggests still another solution, teaching use of a major ingredient expressed by the formula, (Ba.sub.k-x M.sub.x)O.sub.k TiO.sub.2, where M is at least one of Mg, Zn, Sr and Ca. This major ingredient is admixed with boric oxide (B.sub.2 O.sub.3), SiO.sub.2 and, optionally, at least one other metal oxide selected from among BaO, MgO, ZnO, SrO and CaO.
A further solution is found in Wada et al. U.S. Pat. No. 4,610,968, which proposes ceramic compositions including a major ingredient expressed by the formula, (Ba.sub.k-x M.sub.x)O.sub.k TiO.sub.2, where M is at least one of Mg, Zn, Sr and Ca. This major ingredient is admixed with B.sub.2 O.sub.3 and at least one metal oxide selected from among BaO, MgO, ZnO, SrO and CaO.
All the foregoing known compositions make possible the fabrication of ceramic bodies by firing at temperatures of not more than 1200.degree. C. in a nonoxidative (reductive or neutral) atmosphere. The ceramic bodies may therefore be cosintered with electrodes of a base metal such as nickel. The resulting capacitors have specific dielectric constants of not less than 2000, and the temperature dependencies of their capacitances are within plus or minus 10 percent in a temperature range of -25.degree. to +85.degree. C.
While these values are satisfactory for all practical purposes, we have nevertheless been hard pressed by our customers, with the recent development of microelectronics, for ceramic capacitors that have higher specific dielectric constants with no less temperature dependences.